1. Field of the Invention
The present invention relates to an apparatus and a method for suppressing growth of an oxide film on a coil of a rolled strip which has been wound from hot rolling equipment onto a down-coiler and stored in a coil yard.
2. Description of the Related Art
FIGS. 13(a) and 13(b) schematically show a general state of storage of the coils. FIG. 14 is a graph showing the thickness of an oxide film, in the width direction, on the coil in a conventional state of storage.
A rolled strip after rolling in hot rolling equipment is wound spirally by a down-coiler, carried to a coil yard, and stored there. The posture of the coil in storage is a vertical posture of a coil C as shown in FIG. 13 (a), or a horizontal posture of the coil C as shown in FIG. 13(b). In this posture of storage, the coils are placed in a layer, or stacked in layers, on a frame (not shown). The coils stored in that posture are cooled by natural ventilation or by forced cooling in a building for about 110 hours in summer, or about 80 hours in winter.
In the above posture of coil storage, the rolled strip is cooled in the coil yard while being left to stand for a long time at a high temperature in an oxidizing atmosphere of the air. Thus, an oxide film grows on the surface of the coil over a long time. In the case of the coil of the rolled strip spirally wound, both of its edges directly touch the air. From the edges, air penetrates the gap between the adjacent turns of the rolled strip. Consequently, the oxide film growing on the strip surface of the coil differs in thickness across its width direction.
The graph indicated in FIG. 14 shows the thickness of the oxide film in the width direction of the coil. During rolling, the oxide film is formed uniformly to a thickness of 8 xcexcm, and then the coil is cooled for a predetermined period of time in the coil yard. As shown in this graph, the oxide film on the strip surface of the coil grows to a thickness of 18 xcexcm at the edge, 15 xcexcm at a distance of 50 mm from the edge, 9 xcexcm at a distance of 300 mm from the edge, and 8.5 xcexcm at a distance of 470 mm from the edge (i.e., at a central position in the width direction). One will see that when the rolled strip in the coiled form is cooled, the oxide film grows to a thickness of 8.5 xcexcm at the central position in the width direction, but to a thickness of 18 xcexcm, a value more than two-fold, at the edge.
The rolled strip having the oxide file formed on the surface thereof is generally acid pickled to remove the oxide film. Then, the rolled strip is transported to a subsequent step such as a plating line. The acid pickling of the rolled strip is performed by guiding the rolled strip into an acid pickling tank holding an acidic liquid, and immersing the rolled strip therein for a predetermined time, thereby cleaning off the oxide film formed on the surface. That is, the duration of immersion of the rolled strip in the acid pickling tank is set in accordance with the thickness of the oxide film formed on the surface of the rolled strip. Hence, the rolled strip, which has the oxide film 8.5 xcexcm thick at the central position and 18 xcexcm at the edge, needs to be immersed in the acid pickling tank for a long time adapted for the oxide film which is 18 xcexcm in thickness. As a result, the transport speed of the rolled strip in the acid pickling is slowed, decreasing the overall production efficiency. When the thickness of the oxide film formed on the surface of the rolled strip is great, moreover, the consumption of the acidic liquid used increases, raising the pickling cost.
The present invention has been accomplished to solve the above-described problems with the earlier technologies. It is an object of this invention to provide an apparatus and a method for suppressing growth of an oxide film on a coil of a rolled strip by properly storing and cooling the coil, while achieving an increase in productivity.
An aspect of the invention, as a means of attaining the above, object, is an apparatus for suppressing growth of an oxide film on a coil which is a spirally wound, rolled strip, wherein covers are provided for covering at least opposite sides of the coil.
Thus, the opposite sides of the coil are cooled while being shielded from the outside air. Penetration of air into the interior of the coil is inhibited, the growth of the oxide film on the rolled strip (coil), especially at its edge, is suppressed, and the formation of an extremely thick oxide film at the edge as compared with the center in the width direction can be prevented. As a result, the coil has a thin oxide film overall, and widthwise is leveled in thickness. The acid pickling time for cleaning off the oxide film can be markedly decreased, and the transport speed of the rolled strip during acid pickling can be increased, whereby the overall production efficiency can be raised. The consumption of the acidic liquid used can also be decreased to reduce the pickling cost.
In the apparatus for suppressing growth of an oxide film on a coil, a heat insulating material may be provided on a surface of each of the covers in intimate contact with the coil. By this measure, rapid local cooling of the coil is prevented, and penetration of air into the interior of the coil is reliably inhibited. Consequently, the formation of an extremely thick oxide film at the edge compared with the center in width direction can be prevented. The resulting oxide film is thin overall, and widthwise is leveled in thickness
In the apparatus for suppressing growth of an oxide film on a coil, each of the covers may be shaped like a ring having a through-hole at a center thereof, and eye rings formed to project toward the through-holes of the covers may be connected together and fixed by bolts and nuts. By these measures, the covers can be mounted easily on the coil.
In the apparatus for suppressing growth of an oxide film on a coil, a cooling water path may be provided in each of the covers. By this measure, the coil is forcibly cooled with cooling water, with its opposite sides being shielded from the outside air. Consequently, the cooling time can be shortened, and the storage time of the coil in the coil yard can be reduced.
In the apparatus for suppressing growth of an oxide film on a coil, each of the covers may be shaped like a disk having a flange portion in an outer peripheral area of the cover, and the cooling water path may be provided along a circumferential direction of the cover. Since the cooling water path becomes long, the cooling efficiency can be increased.
In the apparatus for suppressing growth of an oxide film on a coil, the covers may be provided with a supply path and a discharge path for supplying and discharging an inert gas into and from a hollow portion of the coil. By this measure, the opposite sides of the coil are shielded from the outside air, and the hollow portion of the coil is supplied with the inert gas. Thus, penetration of air into the interior of the coil is inhibited, and the oxidizing atmosphere inside the coil is purged with the inert gas. Consequently, growth of the oxide film on the rolled strip can be suppressed reliably.
In the apparatus for suppressing growth of an oxide film on a coil, each of the covers may be shaped like a disk having a flange portion in an outer peripheral area of the cover, and the supply path and the discharge path for the inert gas which communicate with the hollow portion of the coil may be provided in the center of the covers. By this measure, a simple constitution can increase the cooling efficiency.
In the apparatus for suppressing growth of an oxide film on a coil, the covers may be a cup for accommodating the coil and a cup for covering an opening of the cup. By so doing, the coil can be cooled, with its whole being shielded from the outside, so that the growth of the oxide film on the rolled strip can be suppressed reliably.
Another aspect of the invention is a method for suppressing growth of an oxide film on a coil which is a spirally wound, rolled strip, comprising cooling the coil while covering at least opposite sides of the coil with covers.
Thus, penetration of air into the interior of the coil can be inhibited, the growth of the oxide film on the rolled strip (coil), especially at its edge, can be suppressed, and the formation of an extremely thick oxide film at the edge compared with the center in the width direction can be prevented. As a result, the coil has a thin oxide film overall, and widthwise is leveled in thickness.
The method for suppressing growth of an oxide film on a coil may comprise cooling the coil while the heat insulating surfaces of the covers and the coil are in intimate contact with each other. By so doing, rapid local cooling of the coil is prevented, and penetration of air into the interior of the coil is reliably inhibited. Consequently, the formation of an extremely thick oxide film at the edge compared with the center in width direction can be prevented. The resulting oxide film becomes thin overall and widthwise is leveled in thickness.
The method for suppressing growth of an oxide film on a coil may comprise cooling the coil by flowing cooling water in a cooling water path provided in each of the covers. By so doing, the coil is forcibly cooled with cooling water, with its opposite sides being shielded from the outside sir. Consequently, the cooling time can be shortened, and the storage time of the coil in the coil yard can be reduced.
The method for suppressing growth of an oxide film on a coil may comprise cooling the coil while supplying and discharging an inert gas into and from a hollow portion of the coil. By so doing, the opposite sides of the coil are shielded from the outside air, and the hollow portion of the coil is supplied with the inert gas. Thus, penetration of air into the interior of the coil can be inhibited, and the oxidizing atmosphere inside the coil can be purged with the inert gas. Consequently, growth of the oxide film on the rolled strip can be suppressed reliably.
The method for suppressing growth of an oxide film on a coil may comprise cooling the coil in an accommodated state. By so doing, the coil can be cooled, with its whole being shielded from the outside, so that the growth of the oxide film on the rolled strip can be suppressed reliably.